Field of the Invention
The present invention relates to a communication device/method using a communication protocol including a transport layer. The TCP/IP (Transmission Control Protocol/Internet Protocol) represents the communication protocol including a transport layer. An example of a conventional TCP/IP network system of the conventional art is shown in FIG. 1.
In the network system, as shown in FIG. 1, a network 610 and a different network 620 are connected to each other through an IP router 150 so that terminals 131 and 132 belonging to the network 610 and terminals 141, 142 and 143 belonging to the network 620 may communicate with each other. The IP router 150 receives an IP datagram from the interface of an IP address a.a.a.1 or a.a.b.1, refers to a routing table by using a destination IP address of the IP header of the IP datagram as a key, and outputs the IP datagram to a proper interface. For example, the IP datagram, as transmitted from the terminal 131 of an IP address a.a.a.2 to the terminal 141 of an IP address a.a.b.2, is received by the a.a.a.1 interface of the IP router 150 and is outputted to the a.a.b.1 interface to reach the terminal 141.
Noting that a number of downloads are performed of data from a server terminal to client terminals in many multimedia services utilizing networks, there has been recently developed an asymmetric access network in which a bandwidth of inputs to client terminals is extremely wider than that of outputs from client terminals.
In this network having the asymmetric transmission speed, however, the throughput of the TCP in the direction from the server to the clients may be lowered, because the recommended TCP implementation method has an algorithm "at least one ACK (Acknowledgment) is returned for two TCP segments". This algorithm determines the effective bandwidth from the server to the clients in the following manner.
The time period for the ACK to be transmitted is the division of the bandwidth (Vup) in the direction from the clients to the server by the ACK size (Sack). For this time period, the data of 2.times.TCP maximum segment size (mss) can be transmitted. The bandwidth (Vdown) in the direction from the server to the clients is expressed by Vup.times. (2.times.mss)/Sack and cannot be exceeded. Thus, the throughput in the direction from the server to the clients has this upper limit.
If, therefore, the algorithm that one ACK is returned for two TCP segments is changed to return one ACK for N-number (N&gt;2) of TCP segments thereby to reduce the number of ACKs, the throughput may be improved, but this breaks the recommended TCP implementation method. Moreover, it is difficult to change the TCP implementation for all terminals.
More generally, the TCP segment is called the transport layer protocol data unit, and the IP datagram is called the network layer protocol data unit.